Numerous prior art solutions address the problem of improving the ability of robots and other vehicles to surmount obstacles and/or right themselves following a rollover accident. For example, US Patent Application US 2010/0139995 A1 “Mobile Robotic Vehicle” describes a tracked mobile robot featuring one or two articulated arms mounted external to the vehicle. These arms are rotatable about an axis located rearward of the center of gravity of the robot chassis and configured to trail the robot. The arms are actuated to effect endwise rollover of the vehicle by, for example, rotating to raise the rearward end of the robot and invert the robot endwise. In such prior art solutions involving external appendages, the employed locomotion mechanisms may be difficult to operate autonomously or by remote control, because the sequence of actions may not be easily performed autonomously or remotely by a human operator. The external arms are subject to snagging or fouling by debris or vegetation in the environment. Further, the arms may be prevented from moving towards a desired position to effectuate a particular maneuver by external obstacles in their paths.
In another prior art example, U.S. patent application No. US 2009/0192674 A1 discloses a hydraulically propelled, gyroscopically stabilized motor vehicle incorporating a double gimbal control moment gyroscope (CMG) said to prevent the vehicle from overturning. However, the US 2009/0192674 A1 prior art solution has serious deficiencies. The prevention of overturning is dynamically similar to rotating the vehicle from one gravitationally stable state (e.g., on its side) to another (upright). In the US 2009/0192674 A1 prior art, the CMG is an energy wheel comprising a wheel spinning in the horizontal plane, gimbaled in the pitch axis of the vehicle and gimbaled and elastically mounted in the roll axis. The wheel of the CMG is referred to as a gyroscopic wheel and is a conventional flywheel claimed to be capable of operating at the required rotational speed. The pitch axis gimbal is connected to the roll axis gimbal, i.e., the CMG is double gimbaled in the conventional sense. US 2009/0192674 A1 claims that the double gimbal CMG functions to prevent a vehicle rollover accident. This requires torque magnitude comparable to that required to effect rotation of the vehicle from lying on its side to an upright attitude, i.e., rotation from one gravitationally stable state to another. US 2009/0192674 A1 states that an object of the vehicle disclosed therein is to provide a standardized platform upon which any appropriately configured cargo, including a conventional passenger car body may be mounted. The weight and dimensions of any practical vehicle dictate that the torque required to prevent vehicle rollover must be relatively large, which places extremely challenging requirements on the CMG. As would be well known to those skilled in the art, the double gimbal CMG is a severely disadvantageous configuration for producing high levels of torque, because some part of the output torque of a double gimbal CMG must be balanced by the gimbal motors. This means that the output torque is limited by the motor torque limit—a very serious constraint in practice. An additional drawback to the US 2009/0192674 A1 prior art vehicle is the use of only one CMG. If the vehicle begins to rotate about its roll axis toward a rollover, the CMG rotor spin axis tilts about the vehicle pitch axis such that the CMG produces a roll torque component to resist the vehicle rollover. However, as the spin axis tilts, it also produces a yaw torque component, which may actually increase the rollover tendency if the vehicle is in a turn, or may otherwise contribute to vehicle instability.
International Application document WO 2011/017668 A2 discloses a spherical robot incorporating an internal cubical frame populated with four single-gimbal CMGs, with each gimbal axis at an angle on each face. The robot is not limited to spheres as an outer structure, but to all generalized amorphous ellipsoidal configurations as well, i.e., those having axially symmetric surfaces with circular cross sections. The CMGs may be operated individually or simultaneously to effect the desired function, such as rolling, steering, stationary rotation around the contact point or balancing in position. The angular momentum stored in the CMGs may be utilized to achieve rapid acceleration or deceleration in any direction. The center of mass of the robot is fixed to the center of the sphere.
The prior art spherical or amorphous ellipsoidal vehicle disclosed in WO 2011/017668 A2 is not capable of continuous uninterrupted travel in any particular direction, is not gravitationally stable on a sloped underlying surface, and its slope climbing capability is severely limited, for several reasons. A CMG produces output torque on a device by exchanging momentum with it. This momentum exchange is accomplished by tilting the CMG rotor spin axis at some angular rate. A CMG or system of CMGs has a maximum amount of angular momentum that can be stored or dispensed, dictated by their design and configuration. The CMGs cannot continuously produce output torque in a given direction, because the maximum stored momentum capacity will eventually be reached. Therefore, CMGs utilized to apply torque to drive a spherical vehicle must be periodically reset to or toward a net zero configuration by means of an external torque. Possible sources of external torque include a gravity-induced torque on an internal offset mass, or an external mechanism that can push off of the underlying surface. Since the CMG output torque magnitude is proportional to the product of the rotor angular momentum and the spin axis tilt rate, the CMGs must be returned to the net zero configuration slowly to avoid applying undesirably large torque opposite the desired direction. The prior art spherical or amorphous ellipsoidal vehicle disclosed in WO 2011/017668 A2 does not teach of any source of external torque for resetting the CMGs, since it lacks any external appendage, and the center of mass is strictly at the geometric center of the sphere. This means the vehicle can only generate a drive torque and accelerate in a given direction for a limited period of time before saturating the CMGs. With no way to effectively reset the CMGs back to a neutral state, the vehicle can now only generate torque and accelerate in the opposite direction. This vehicle oscillatory motion is a consequence of the conservation of momentum, and clearly illustrates one of the critical shortcomings of the WO 2011/017668 A2 prior art vehicle.
The fact that the center of mass is fixed at the geometric center of the WO 2011/017668 A2 prior art vehicle, plus the vehicle can only generate torque in a given direction for a limited time means that this vehicle cannot remain stationary on a slope for very long because there is no way to continuously counterbalance the gravity torque on the vehicle about the point of contact with the sloped surface.
A second serious deficiency of the prior art vehicle disclosed in WO 2011/017668 A2 arises from the fact that the center of mass is fixed at the center of the vehicle. This becomes a handicap when the vehicle is to surmount an obstacle such as a step, because the torque required to rotate the vehicle to surmount the obstacle is greater than if the vehicle center of mass could be shifted toward the obstacle to reduce the moment arm through which the vehicle weight acts. The required higher torque dictates that the CMG angular momentum be higher and/or that the rotor spin axis tilt rate be higher. These increased requirements mean the CMGs must be heavier or spin faster with consequently lower safety factor and/or that the tilt actuators be more powerful, and therefore heavier and more costly. The heavier CMGs increase the vehicle weight and therefore the CMG torque requirement.
U.S. patent application Ser. No. 12/619,582 discloses the use of a pair of CMGs to augment the drive torque produced by a pendulum mass-shifting drive system in a spherical vehicle. In the spherical vehicle disclosed in that application, the pair of CMGs produce a resultant torque about only a single axis parallel to the axis supporting the pendulum, such that they contribute to only forward and rearward propulsion of the vehicle. Steering of the disclosed spherical vehicle is provided by swinging the pendulum to one side while also swinging forward or rearward, causing the vehicle to travel in an arc to the left or right. A key feature of some embodiments of the present invention is the use of CMG-generated yaw torque for steering, i.e., changing the heading of the vehicle while stationary or while moving forward or backward. This eliminates the need to swing the pendulum to either side for steering, an approach requiring that the volume within the vehicle on either side of the pendulum be clear of any other components to allow for the sideward swing of the pendulum. An additional shortcoming of the Ser. No. 12/619,582 prior art vehicle is that it has only a single gravitationally stable state on a sloped underlying surface.